Effects of Silica Modification (Mg, Al, Ca, Ti, and Zr) on Supported Cobalt Catalysts for H2-Dependent CO2 Reduction to Metabolic Intermediates

Serpentinizing hydrothermal systems generate H2 as a reductant and harbor catalysts conducive to geochemical CO2 conversion into reduced carbon compounds that form the core of microbial autotrophic metabolism. This study characterizes mineral catalysts at hydrothermal vents by investigating the interactions between catalytically active cobalt sites and silica-based support materials on H2-dependent CO2 reduction. Heteroatom incorporated (Mg, Al, Ca, Ti, and Zr), ordered mesoporous silicas are applied as model support systems for the cobalt-based catalysts. It is demonstrated that all catalysts surveyed convert CO2 to methane, methanol, carbon monoxide, and low-molecular-weight hydrocarbons at 180 °C and 20 bar, but with different activity and selectivity depending on the support modification. The additional analysis of the condensed product phase reveals the formation of oxygenates such as formate and acetate, which are key intermediates in the ancient acetyl-coenzyme A pathway of carbon metabolism. The Ti-incorporated catalyst yielded the highest concentrations of formate (3.6 mM) and acetate (1.2 mM) in the liquid phase. Chemisorption experiments including H2 temperature-programmed reduction (TPR) and CO2 temperature-programmed desorption (TPD) in agreement with density functional theory (DFT) calculations of the adsorption energy of CO2 suggest metallic cobalt as the preferential adsorption site for CO2 compared to hardly reducible cobalt–metal oxide interface species. The ratios of the respective cobalt species vary depending on the interaction strength with the support materials. The findings reveal robust and biologically relevant catalytic activities of silica-based transition metal minerals in H2-rich CO2 fixation, in line with the idea that autotrophic metabolism emerged at hydrothermal vents.


Catalytic Testing for CO2 Hydrogenation
Catalytic performance testing at 20 bar and typically 180°C was performed in a fixed-bed reactor setup shown schematically in Figure EM1. The setup is equipped with mass-flow controllers (MFCs, Bronkhorst) to feed H2 (Air Liquide, 99.999%), and the pre-mixed reactant gas mixture (30 vol% CO2, 60 vol% H2, 10 vol% Ar as internal standard, Air Liquide). The gas stream was first passed through a stainless steel (316L grade) capillary loop (length, l = 300 mm, inner diameter, i.d. = 3 mm) at 200°C to pre-heat the reactant gas mixture. The fixed-bed stainless steel (316L grade) micro-reactor (i.d. = 12 mm) was heated by two finned copper elements (1) with two embedded 350 W heating cartridges. The temperature was controlled by two thermocouples (K-type, 0.5 mm, (2)) placed at the start and end of the catalyst bed. The catalyst bed (5) typically consisted of 850 mg of the Co-based catalyst (sieve fraction 200-400 µm grain size) diluted by 6.2 cm 3 of SiC powder (Alfa Aesar, 46 grit) for improved heat transfer. Upstream of the catalyst bed, a layer of 3.4 cm 3 of SiC powder (Alfa Aesar, 46 grit, (4)) was added for further pre-heating of the reactant gas mixture and to establish a plug-flow behavior before entering the catalyst bed. The free volume inside the reactor was reduced by two stainless steel spacers (316L grade, l = 58 mm and l = 62 mm) separated from the catalyst bed by two quartz wool plugs (3). Downstream of the reactor, the product gas stream was passed through two consecutive cold traps set to 50°C and 100°C at the reaction pressure to condense higher boiling oxygenate products and water. All further downstream gas lines were heated to 170°C to prevent condensation of reaction products. When depressurized after the dome pressure regulator, the gaseous products were analyzed by an online gas chromatograph (GC, modified Agilent 7890B). The GC was equipped with two sampling loops. One loop fed into a capillary column (Restek RTX-1, 60 m) with an flame ionization detector (FID) and the other one into two consecutive packed-bed columns (HS-Q 80/120, 1 m + 3 m) equipped with a thermal conductivity detector (TCD) for the analysis of H2, CO2 and C2-3 hydrocarbons. An additional TCD was used to detect Ar, CH4 and CO, separated by a molecular sieve column (MS-5A 80/120, 3 m) along the same analysis channel. CO2, CH4, and CO were quantified by TCD response factors relative to Ar. CO2 conversion (XCO 2 ) and selectivities (Si) were calculated from the following equations: ACO 2, AAr represent the peak areas of CO2 and Ar from the TCD during the reaction and A 0 CO 2, A 0 Ar the peak areas of CO2 and Ar from the TCD during a blank measurement.
In this equation, ni represents the molar flow of product compound i and xi the carbon number of the compound. The molar product flows were calculated from the TCD peak areas using the corresponding response factor 1 .

Figure EM1
Scheme flow sheet of the fixed-bed flow reactor system with online GC used to perform CO2 hydrogenation experiments.

Figure EM2
Representative online gas product chromatograms for CO2 hydrogenation with 10 wt % Co/SBA-15. Short-chain hydrocarbon products and methanol were detected with the flame-ionization detector (FID). Permanent gases such as Ar, CH4, and CO were detected on a first thermal conductivity detector (TCD), while CO2 and water were detected on a second TCD.

Details of the Co20/SiO2 Model
A Co20 cluster supported on the amorphous silica slab (Co20/SiO2 model) was adopted as the theoretical model of the Co/SBA-15 catalyst. The transition metal cluster was supported on the silica surface by forming -O-SiO2 bonds with surface -OH (silanol) groups as reported in a previous theoretical study, 2 and the H2-TPR signals at T > 600 ℃ in the present study are also indicative of such interactions. Thus, the Co20 was placed right above each surface -OH group with removing an H atom, and the relative stability of each structure was compared. Before the geometry optimization, the Co20 was rotated to maximize the number of Co-O-SiOx bonds and H atoms were removed from the surface -OH groups within 3 Å distance from cobalt atoms. If other -OH groups interacted with cobalt (i.e., the Co-OH length became < 3 Å) during the geometry optimizations, H atoms were also removed from those -OH groups and the geometry was re-optimized. Based on this procedure, 32 structures (= the number of surface -OH groups) were generated, and the most stable one was adopted as the Co20/SiO2 model are the potential energies of the isolated H2, Co20 cluster and silica slab, respectively. The number of the removed H atoms from the silica slab is denoted as n.

Figure CM1
Relative stability of the Co20 cluster on each interaction site in the silica slab. The most stable structure (No.23) is pointed out by the black arrow. Note that one surface -OH group is located below the other -OH groups, and thus, the Co20 cluster located at that site (No.18) could not be obtained due to the steric repulsion.

CO2 Adsorption at Co−O−MOx (M = Ti or Zr)
To investigate the CO2 adsorption at Co-O-MOx (M = Ti or Zr), a Si atom in the Co20/SiO2 model was replaced by Ti or Zr because those metals tend to be in the +Ⅳ oxidation state in silica as reported in previous studies. 3,4 As shown in Figure 4d, e and Figure CM3, four SiO2 units are directly bonded to the Co20 cluster. Thus, a Si atom in one of those four SiO2 units was replaced by a Ti or Zr atom, and the most stable structures were adopted as the theoretical models for the Co/Ti-SBA-15 and Co/Zr-SBA-15 catalysts, respectively. All of the possible adsorption structures of CO2 around the Co-O-MOx site were investigated, and the most stable one is shown in Figure CM4.

Comparison between the Co20/SiO2 and Co55/SiO2 Model
The adsorption energy of CO2 with the Co20 cluster was compared with a Co55 cluster, which corresponds to a 1 nm size cobalt particle. Although Co55 is also smaller than the experimental particle size (e.g. 8.8 nm for the 10 wt % Co/SBA-15 catalyst), from this range of size, the adsorption energy of small molecules (e.g. HCOOH) is almost constant against the size of the cobalt cluster. 5 The icosahedron Co55, which was reported as the most stable structure in the gas phase by previous studies, 6, 7 was adopted as the initial structure of the model. The Co55 cluster was placed on the silica slab model at the same interaction site adopted in the Co20/SiO2 model. H atoms were removed from the surface -OH groups before and during the geometry optimizations to form the Co-O-SiOx bonds as performed for the Co20/SiO2 model. By using the obtained Co55/SiO2 model, adsorption structures of CO2 on the neutral and positively charged cobalt species were investigated. For the investigation of CO2 on the neutral species, we focused on the adsorption sites around the neutral charged cobalt atom at the largest distance from the silica surface because a lot of neutral cobalt is present in the Co55 cluster. The most stable adsorption structure among our investigations is shown in Figure CM5. The adsorption energy on the neutral species was stronger than that of the positively charged cobalt species by 0.15 eV. Thus, both Co20 and Co55 cluster models showed the same trend for the CO2 adsorption on the Co 0 and Co δ+ species although the Co20/SiO2 model tends to overestimate the adsorption energy in comparison with the Co55/SiO2 model. Note that any dynamical reconstructions of the materials under the experimental condition are not considered in our model. Additionally, we focus on the amorphous silica surface without pore structure as the local structure of SBA-15.